The present invention relates to a method of electrophotographic wet reversal development and in particular, to a method of electrophotographic wet reversal development by which overall uniform development with little edge effect and with excellent tone reproduction can be carried out.
An electrophotographic photoreceptor used as a printing plate is obtained by coating a photoconductive material dispersed or dissolved in a binder on an aluminum sheet subjected to sand blasting and anodizing as a support.
The electrophotographic photoreceptor is made into a printing plate through a charging step, an exposing step, a developing step, and a fixing step or, for some construction of photoreceptor, is made into a printing plate through a dissolving-out step and a gumming step after the fixing step as mentioned, for example, in Japanese Patent Kokoku Nos. 37-17162, 38-7758, and 46-39405 and Japanese Patent Kokai Nos. 52-2437, 57-161863, 58-2854, 58-28760, 58-118658, 59-12452, 59-49555, 62-217256, 63-226668, and 1-261659.
Plate making machines for obtaining printing plates include so-called contact printing system in which an original and an electrophotographic photoreceptor brought into close contact with each other are exposed, so-called projection system in which an original is projected to an electrophotographic photoreceptor through a lens, and scanning exposing system in which an image information of an original is converted to electric signal and an electrophotographic photoreceptor is exposed thereto with laser beam. All of these systems involve positive exposure and negative exposure and there are normal development and reversal development for development.
In general, the reversal development in electrophotography causes larger edge effect than the normal development. In order to diminish the edge effect, it is effective to put a developing bias electrode close to an electrophotographic photoreceptor, but since a bias voltage is applied to the developing bias electrode, there is the fear of shortcircuiting, and the electrode cannot be so close to the photoreceptor. Furthermore, since the distance between the developing bias electrode and the electrophotographic photoreceptor is short, there may occur failure in transportation or there may be caused damage of the surface of the electrophotographic photoreceptor. Thus, the distance between the developing bias electrode and the electrophotographic photoreceptor is usually set at 1-5 mm. Length of developing bias electrode of conventional electrophotographic wet developing apparatuses is set about 100-300 mm.
Another method for diminishing the edge effect is to raise bias voltage, but according to this method, the negative portion of dots is destroyed and the resulting image is poor in tone reproduction.
The developing bias voltage of conventional electrophotographic wet developing apparatuses is set at several ten percents of the surface potential of the electrophotographic photoreceptor. When the developing bias voltage is set at such level, because the surface potential of photoreceptor is higher than the developing bias voltage in the non-image portion, the toner particles in the developer are developed on the developing bias plate and a mass of the developed toners generate a voltage. Since the sum of the voltage generated from this mass of toners and the developing bias voltage is applied to the electrophotographic photoreceptor as a new developing bias voltage, there also occur the problems of obliteration of negative portion of dots and of the image becoming bigger.
Furthermore, when toner is electrically deposited in a large amount on the developing bias electrode, the developing bias voltage is not normally applied to cause troubles in images and so, the toner deposited on the electrode must be periodically removed and a large manpower must be consumed for maintenance.